The present invention relates to an encapsulation mold to be used when small electric elements such as semiconductor elements or other kinds of electric components are encapsulated by thermosetting resin.
A conventional example of the aforementioned encapsulation mold will be first described with reference to FIGS. 19-24.
FIG. 19 is a plan view of a lower retainer plate 50 of the mold. A center plate 41 with a cull portion 42, first runners 43, and second runners 44 is set at the center of the lower retainer plate 50. A third runner 46 at each insertion section 49 of the lower retainer plate 50 has a runner entrance 45 opened at an exit of the second runner 44. The third runner 46 has gate portions 47 and package portions 48.
FIG. 20 is a sectional view of the lower retainer plate 50 of FIG. 19 taken along the line XX--XX, more specifically, the left side from the center of the diagram shows a sectional view of the cull portion 42 and the right side is a sectional view of the second and third runners 44, 46. FIG. 21 indicates the in use state of the encapsulation mold. An upper retainer plate D is secured to a fixed side of a press (not shown) and a pot C for heating inserted resin D is placed at the center of the upper retainer plate C. A plunger B presses out the resin A heated in the pot C.
The lower retainer plate 50 is mounted at a vertically movable part of the press and moved up and down so that the cull portion 42 of the center plate 41 is coincident in position with the pot C. After the lower retainer plate 50 is pressed to the upper retainer plate D, the resin A is injected and molded, and after molding, the lower retainer plate 50 is detached from the upper retainer plate D. A molded product is separated from each of the package portions 48 of the lower retainer plate 50.
FIG. 22 is a sectional view along line XXII--XXII of FIG. 19 showing the first runner 43, FIG. 23 is a sectional view along line XXIII--XXIII of FIG. 19 showing the second runner 44, and FIG. 24 is a sectional view along line XXIV--XXIV of FIG. 19 showing the third runner 46.
The operation of the conventional mold will be described with reference to FIGS. 19-24.
Referring first to FIG. 21, the lower retainer plate 50 is moved upward to make the cull portion 42 of the center plate 41 agree with the pot C in position, and is then pressed against the upper retainer plate D. While the pot C is empty, the plunger B is raised and both the upper and the lower retainer plates D, 50 are heated to a setting temperature of the thermosetting resin by a built-in heater (not shown).
Subsequently, a block of the resin A which has been pre-heated in a preceding process is inserted in the pot C, and the plunger B the temperature of which has been preliminarily raised is moved down to press the inserted resin A. When the resin A is heated and melted by the conduction of heat from the pot C, plunger B, and cull portion 42, the plunger B impresses an injection force to the melting resin A, so that the resin A is uniformly pushed out from the cull portion 42 to each first runner 43.
The extruded resin A in the melted state is, through the first runners 43, second runners 44, and third runners 46, filled in the package portions 48. The filled resin is started to be thermally set. After a predetermined setting time, the lower retainer plate 50 is lowered and the molded products are taken out from the package portions 48.
In the above-described arrangement of the mold, the resin A is heated by the conduction of heat from the pot C, plunger B, and cull portion 42. However, it takes much time to raise the temperature of the resin A of a small thermal conductivity to attain sufficient viscosity for injection, leading to an increase of the encapsulating time.
Moreover, the resin heated to the encapsulating temperature has lower viscosity and accordingly better fluidity, and then the viscosity is increased and the fluidity is decreased when setting is started. It is desirable to perform the encapsulation while the resin has good fluidity. If many small electric elements are to be encapsulated concurrently in many package portions 48 so as to improve the productivity, the situation requires an extension of the first, second, and third runners 43, 44, 46. The resin becomes disadvantageously set during running in the long runners. For overcoming this disadvantage, it is necessary to enlarge the sectional areas of the runners and shorten the running time of the resin in the long runners. However, this gives rise to another disadvantage that more resin is left in the long runners, thus reducing the efficiency with which the resin is used.
If the sectional areas of the runners are increased to shorten the running time of the resin in the runners, the resin passes therethrough without the heat from the cull portion 42 and the first, second, and third runners 43, 44, 46 being transmitted to the central part of the resin, resulting in the temperature difference between the outer part and the central part of the flowing resin in the runners. The resin of the encapsulated products shows poor characteristics by lacking uniformity and including voids and the like.